Source : Webster's Revised Unabridged Dictionary (1913)

Grimace \Gri*mace"\ (gr[i^]*m[=a]s"), n. [F., prob. of Teutonic
   origin; cf. AS. gr[imac]ma mask, specter, Icel. gr[imac]ma
   mask, hood, perh. akin to E. grin.]
   A distortion of the countenance, whether habitual, from
   affectation, or momentary aad occasional, to express some
   feeling, as contempt, disapprobation, complacency, etc.; a
   smirk; a made-up face. [1913 Webster]

         Moving his face into such a hideous grimace, that every
         feature of it appeared under a different distortion.
                                                  --Addison.
   [1913 Webster]

   Note: ``Half the French words used affectedly by Melantha in
         Dryden's ``Marriage a-la-Mode,'' as innovations in our
         language, are now in common use: chagrin,
         double-entendre, ['e]claircissement, embarras,
         ['e]quivoque, foible, grimace, na["i]vete, ridicule.
         All these words, which she learns by heart to use
         occasionally, are now in common use.'' --I. Disraeli.
         [1913 Webster]

Grimace \Gri*mace"\, v. i.
   To make grimaces; to distort one's face; to make faces. --H.
   Martineau.

Source : WordNet®

grimace
     n : a contorted facial expression; "she made a grimace at the
         prospect" [syn: {face}]
     v : contort the face to indicate a certain mental or emotional
         state; "He grimaced when he saw the amount of homework he
         had to do" [syn: {make a face}, {pull a face}]